Conventionally, gas engines that combust a fuel gas, such as natural gas or town gas, have been mainly used in power generating equipment (see Patent Literature 1, for example). In recent years, in view of various issues such as the crude oil resource amount and exhaust gas regulations, it has been proposed to use a gas engine as a main engine of a ship instead of a diesel engine that uses heavy oil as a fuel.
For example, Patent Literature 2 discloses a 4-stroke gas engine used as a main engine of a ship. The gas engine is connected to a compressor and a turbine of a turbocharger via an intake passage and an exhaust passage. The intake passage is provided with a throttle valve, and a mixer that mixes air with a fuel gas supplied from a fuel flow control valve is provided upstream of the compressor.
In the gas engine disclosed by Patent Literature 2, in order to improve load responsiveness while keeping highly precise air-fuel ratio (excess air ratio) control, when a change occurs in a speed command value signal that is a target value of the engine rotational speed, the fuel gas flow rate controlled by the fuel flow control valve and the degree of opening of the throttle valve are changed. In particular, Patent Literature 2 describes in relation to the fuel gas flow rate that the upper limit value of the fuel gas flow rate may be determined based on the lower limit value of the excess air ratio in order to prevent misfire and abnormal combustion.